FIELD OF THE INVENTION
The invention relates to a burner, particularly for a gas turbine, including a catalytic supporting burner for stabilizing a main burner. The invention also relates to a gas turbine having the burner.
The fuel provided in this case is, for example, natural gas, coal gas or another hydrocarbon-containing and/or hydrogen-containing gas mixture. However, such a mixture or else a fossil fuel in liquid form is also suitable.
The combustion of the fuel mentioned above also gives rise to nitrogen oxides NO.sub.x which are particularly undesirable combustion products. Such nitrogen oxides, along with sulfur dioxide, are the main cause of the environmental problem of acid rain. Consequently, as well as in view of strict statutory norms on limit values for the emission of NO.sub.x, the aim is to keep the NO.sub.x emission of, for example, a burner in a gas turbine particularly low and, at the same time, avoid appreciably influencing the power of the burner or of the gas turbine.
Thus, for example, lowering the flame temperature in the burner has the effect of reducing the nitrogen oxides. In that case, steam is added to the fuel or to a compressed and preheated fresh air, or water is injected into the combustion space. Such measures, which per se decrease the emission of nitrogen oxides from the burner, are referred to as primary measures for the reduction of nitrogen oxides.
Accordingly, all of those measures in which nitrogen oxides contained in the exhaust gas of a combustion process are decreased through the use of subsequent measures, are referred to as secondary measures.
In that respect, the method of selective catalytic reduction (SCR) has gained acceptance throughout the world. In that method, the nitrogen oxides, together with a reducing agent, usually ammonia, are brought into contact with a catalyst and at the same time form nitrogen and water. The use of that technology therefore necessarily entails the consumption of reducing agents. The catalytic converters which are disposed in the exhaust-gas duct for reducing the nitrogen oxides naturally cause a pressure drop in the exhaust-gas duct. However, if the burner is used in a gas turbine, such a pressure drop leads to a considerable power drop in the turbine. Even a power drop amounting to a few points per thousand, in the case of a gas turbine power of 150 MW, for example, and a current-purchasing price of about $0.016/kWh (and about 0.15 DM/kWh in Germany, for example) for current, has a serious effect on the result which can be achieved with such an apparatus.
Recent considerations with regard to the construction of the burner tend towards replacing the diffusion burner or swirl-stabilized premixing burner that is conventionally used in a gas turbine with a catalytic combustion chamber. Lower emissions of nitrogen oxides are achieved with a catalytic combustion chamber than is possible with the above-mentioned burner types. In that way, the known disadvantages of the SCR method (large catalyst volumes, consumption of reducing agent, high pressure loss) can be overcome.
Conventionally, there is provision for using a pilot flame for the stabilization of burners (diffusion burners, swirl-stabilized premixing burners, catalytic burners). The pilot flame is used in order to set a specific starting point for the combustion of the actual main stream of fuel gas. A burner for generating such a pilot flame is conventionally a diffusion burner which constitutes an appreciable source of nitrogen oxides. In view of the environmental problems caused by nitrogen oxides and due to strict statutory conditions for the emission of nitrogen oxides, the aim is therefore to avoid any nitrogen oxide source even as small as that or at least to decrease its emission of nitrogen oxides.